In recent years there has been a growing interest in replacing inefficient thermionic cathodes in electronic devices with much more efficient high field emission structures. A thermionic cathode causes electrons to be emitted from a cathode surface by raising the cathode surface to a very high temperature, either by passing a high electrical current through the cathode itself or through the use of a separate heater. Thermionic emission is very inefficient. Most of the energy is wasted as heat.
Field ionization and emission structures can emit electrons without being maintained at high temperatures. There are many potential applications for such structures. For example, arrays of field emission structures have been proposed for use as cathodes in flat panel displays, field-ionization type chemical detectors, vacuum microelectronic devices, electron microscopes, and high performance high frequency vacuum tubes.
Field emission structures comprise extremely sharp points (typically less then 100 nanometers in radius) of electrically conductive material. High electric fields may be achieved at the points by applying an electrical potential to the points relative to an adjacent structure. Where the point is negatively charged the electric field at the point can be sufficient to discharge large numbers of electrons from the point. When the point is positively charged then the electric field in the vicinity of the point can be sufficient to ionize gas molecules in contact with the point by causing electrons to tunnel through the potential barrier into the point.
The emission or ionization of a point may be controlled by providing a gate electrode structure adjacent the point and applying an electrical potential to the gate electrode. The effect of the potential on the gate electrode on emission or ionization at the point increases as the distance between the point and the gate electrode is reduced. Minimum gate electrode potentials can most conveniently be obtained by fabricating the gate electrode together with the field ionization and emission structures.
Gray et al., U.S. Pat. No. 4,307,507, describe a method for manufacturing field emission cathode sources. The method involves applying a mask to an oriented wafer of single crystal material and then performing an orientation-dependent etch through holes in the mask to produce pits. The orientation dependent etch produces pits which have a pyramidal shape. The pits are the inverse of the desired shape of the points. The mask is removed and a layer of emission material is deposited to fill the pits and cover the surface of the wafer. The wafer is then etched away to expose pointed replicas of the pits, whose sharp points can be used as field ionization or emitters. The Gray et al. method is simple but it does not provide an integrated gate electrode and the points produced are not as sharp as would be desirable.
Nakamoto et al., U.S. Pat. No. 5,483,118, provides a method for fabricating field emission structures with integrated gate electrodes. The gate electrodes are deposited after field emission points are formed. The Nakamoto et al. methods require several separate masking steps. The requirement to align several masks tends to make to Nakamoto et al. methods expensive. It is especially disadvantageous that the Nakamoto et al. methods require aligning features created by masking on one face of a wafer with masks to mask features on the opposite face of the wafer. Furthermore, the Nakamoto et al. methods use equipment that is expensive and not universally available.
Other micro fabrication methods for making microminiature tips are described in Akamine, U.S. Pat. No. 5,580,827, Carver, U.S. Pat. No. 4,916,002, Zimmerman, U.S. Pat. No. 5,334,908, Neukermans et al., U.S. Pat. No. 5,393,647, and, Spindt A Thin Film Field-Emission Cathode, Journal of Applied Physics, V. 47, p. 5248 (1976). Jensen et al., Analytical and semi numerical models for gated field emitter arrays. I. theory, J. Vac. Sci. Technol., B 14(3), (May/June 1996) describes the desired relationship between a gate electrode and a field emitter point.
What is needed is a method for fabricating field-emission structures or arrays of field emission structures that can be practised at lower cost than currently known methods. Also needed is a lower cost method for fabricating field-emission structures or arrays of field emission structures which incorporate a gate electrode. In particular, what is needed is a method for forming a field emission point which is aligned in an aperture of a gate electrode without the necessity of precisely aligning a mask. Also needed is a method to fabricate field-emission points which are sharper than those produced by current methods.